Reprint from my first blog, YogaGuide (http://yogaguide.wordpress.com/2007/10/25/fall-yoga/)
Nadi Shodhana, or Alternate Nostril Breathing balances breath by directing it in & out of each nostril, well, alternately. We know from experience and scads of research on EMDR as well as cross patterning that stimulating the halves of the body alternately assists the nervous system in sorting information. Alternate nostril breathing is also really great for allergies, at least in my case.
Take your right hand and bend in the first and second finger. Place the thumb against the right nostril and inhale smoothly, slowly (say, to a count of four if that’s comfortable) through the left. Close off the left with the ring & pinkie (both nostrils are compressed right now) for a retention of comfortable length (you might try one equal to the count for the inbreath). Release the right nostril and exhale – smoothly and slowly for the same count as the inhale – through the right nostril, and observe a rest, or kumbacha of the outbreath, at the bottom, allowing your spirit to reside in the emptiness, for a comfortable period of time, again you might try another count equal to the inbreath.
Then inhale right nostril, compress it (the left is still compressed from before, so both are closed at the top); practice a comfortable, non-straining retention at the fullness again, release the left nostril and exhale smoothly, yes, for the same count, resting in emptiness at the end of your outbreath.
This was one round and it won’t take as long to do as it does to read about. Do about four rounds to start and add as you become familiar and comfortable.